This invention relates to transilluminators for use with visualizing DNA in agarose gels.
A common method for separating, identifying or purifying DNA from a mixed sample is by electrophoresis of the sample through an agarose gel. The electrophoretic migration rate of DNA through agarose gel is dependent upon the molecular weight of the DNA, as well as such considerations as the agarose concentration and the strength of the electric field. The electrophoresis technique is simple and rapid, and results in the formation of distinct bands of DNA within the gel.
After electrophoresis for a sufficient period, electrophoresis gels are typically stained to visualize the bands of DNA, often with low concentrations of the fluorescent dye ethidium bromide. Ethidium bromide, which becomes bound-up, or intercalated, between bases of the DNA, has an increased fluorescent yield, as compared to free ethidium bromide in solution. UV radiation is absorbed by bound ethidium bromide dye and re-emitted in the red-orange region of the visible spectrum. Thus, the location and relative amount of DNA is detectable by direct examination of an ethidium bromide-stained electrophoresis gel under illumination by ultraviolet light.
One apparatus used to illuminate electrophoresis gels produces ultraviolet light in a closed box. A light source within the box transmits UV light through a horizontal window provided in the top of the box. A gel is positioned over the window for illumination by the UV light, which passes through the gel from the window below. Such an apparatus is referred to as a transilluminator.
The window of the transilluminator typically comprises a purple filter glass centered directly over the UV light source. This filter glass blocks all light except that within a narrow range centered around the specific UV region which causes the fluorescence of ethidium bromide bound to DNA. The filter glass is a relatively expensive component of the transilluminator. Physical damage to the glass can occur when researchers cut out DNA-containing bands of gel while the gel is illuminated on the glass, or can be the result of accidental contact such as from dropping small objects onto the top of the transilluminator. Such damage to the glass surface can affect background UV transmission through the glass. Problems with the glass surface can also arise from permanent staining or contamination by laboratory fluids, gel fragments or residual DNA from gels, which can result in clouding of the glass or in high levels of background fluorescence.
Since the glass surface can easily become damaged from physical contact or contamination, all of which will impair the accurate measurement of DNA within the gels, an inexpensive and easily cleaned protector plate is often placed over the top of the transilluminator. The plate shields the filter glass from accidental physical contact and keeps gels from directly contacting the UV filter during transillumination. There is no significant loss of sensitivity of DNA detection when the protector plate is in place. The protector plate is typically held in place on the surface of the transilluminator by screws, or, if a less permanent mounting is acceptable, by magnetic strips. As the protector plate becomes clouded or damaged, it is replaced, at a far smaller cost than if the filter glass is replaced.
The visualization of ethidium bromide-stained DNA samples on agarose gels with UV light creates a risk to the researcher of eye or skin damage from exposure to the UV radiation. In U.S. Pat. No. 4,657,655, a transilluminator is shown having a cover made of a UV radiation blocking material. This cover greatly reduces the risk of eye and skin damage from UV radiation, thus eliminating the necessity for cumbersome protective equipment for the researcher, such as face shields or eyeglasses.
When such a blocking cover is utilized, the plate protecting the filter glass is permanently mounted on the transilluminator surface by screws set into the housing. These screws allow for replacement of the protector plate as it becomes damaged over time, or even if the researcher simply wishes, for whatever reason, to utilize the transilluminator without the protector plate. The blocking cover is also connected to the top wall of the housing by a hinge which is secured to the top wall by screws which pass through the protector plate covering the top surface of the transilluminator. Thus, to remove the protector plate one must remove both the hinge and blocking cover.